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Posted: May 5, 2008

Discovery paves way for spintronic technology

(Nanowerk News) The weather may be heating up as summer approaches, but thanks to the discoveries of a group of University researchers, computers may soon be cooling down.
Currently, many laptop computers can generate enough heat to allow their users to turn them over and fry eggs on the laptops’ undersides.
In the future, scientists hope to build faster, more powerful computers that do not generate as much heat as modern processing units and won’t cook breakfast, either.
The key to this future generation of computers may lie in the recent discoveries of a team of University scientists who have demonstrated a new instance of the quantum Hall effect — a method of generating voltage — which, unlike modern computer processors, does not generate heat through dissipation of power or electron collisions.
“If you apply a magnetic field to a piece of metal with electrons that can conduct electricity, then you can see that the electrons circle the magnetic field. That is the Hall effect,” said physics professor Zahid Hasan, who led the research team. “If you increase the intensity of the magnetic field, then all of these circles collapse into one great big circle, and electrons only move along the boundary of the metal. This is the quantum Hall effect.”
Most of the different Hall effects discovered in the past require the application of a large magnetic field to generate voltage, and this magnetic field, in turn, generates heat. But the research group found that it was possible to observe a version of the quantum Hall effect without applying any external magnetic field at all.
Decades ago, Princeton physics professor Duncan Haldane GS ’76 suggested a possible model for observing the Hall effect that does not involve applying an external magnetic field. Haldane’s idea was that electrons moving very fast, near the speed of light, could be subject to the laws of relativity in such a way that they would generate their own internal magnetic field.
The researchers recently discovered it was possible to generate such an effect, without the use of an external magnetic field, in bismuth antimony metal alloys. To produce the alloy crystals, however, the physicists needed help from the chemistry department.
“We had to grow a perfect crystal of the metal alloy to allow this [quantum Hall] effect to be clearly visible,” chemistry professor Robert Cava said in an e-mail. “It took a year’s worth of trying different methods and inventing different tricks that you can never read about in books for us to succeed.”
The development of this crystal was another one of the biggest challenges in the series of experiments, postdoctoral research associate Yew San Hor said.
“To grow a high-quality crystal of bismuth antimony, patience and ideas are necessary,” Hor said. “You might think we were lucky, but luck usually was not there. We never gave up, though — this is Rule Number Six in the Cava Lab.”
The Cava Lab’s eight rules also include “Kick butt,” “Don’t be a baby about blowing stuff up” and “Remember: Even a blind squirrel will someday find a nut.”
The researchers did find their nut, or rather their perfect bismuth antimony crystals, but several of the scientists said they were shocked that the alloy was so effective.
“The biggest surprise to me is that we could find such a strange and wonderful electronic state of matter in a material that is as simple as a metal alloy made from two very common elements mixed together,” Cava said.
The bismuth antimony compound exhibited the quantum Hall effect the researchers had been looking for without requiring the application of any external magnetic field. The effect centers on the spin of the metal’s electrons, a phenomenon that is also the basis for the “spintronic” technology that some scientists believe will revolutionize the future of computing.
The spin of an electron can take on one of two values, either up or down. Spintronic technology involves manipulating currents of purely up or down spins. The researchers found that at the surface of the bismuth antimony insulator, they could generate electric fields that naturally separated out the electrons with up and down spins.
More importantly, the spin currents along this surface did not dissipate any heat, a discovery that will help pave the way for future computing technology.
“With this discovery, a new generation of computers is near,” Hor said. Current computer chips are limited in their computing capacities because they generate heat as their transistors become more densely packed, he added. A new kind of technology, based on the team’s discoveries, could overcome that heating problem.
“In modern electronics, the size of devices can be limited by how much heat they give off,” research team member David Hsieh GS said. “With dissipationless currents available, the sizes of devices can be made much smaller, and [they can] be made to run on much less energy.”
The discovery is also an important step toward building better quantum computers, Hasan explained.
Quantum computers could solve certain mathematical problems, such as integer factorization into prime numbers, much faster than current technology, allowing for huge improvements in code-breaking techniques.
Source: The Daily Princetonian (Josephine Wolff)